Fix Node.js warning about wrong module type

# rollup --config rollup.config.js
(node:2833) Warning: To load an ES module, set "type": "module" in the
package.json or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)
[!] RollupError: Node tried to load your configuration file as CommonJS
even though it is likely an ES module. To resolve this, change the
extension of your configuration to ".mjs", set "type": "module" in your
package.json file or pass the "--bundleConfigAsCjs" flag.

Makefile

@@ -33,7 +33,7 @@ BLACK_BIN         = black
 BLACK_OPTS        = --verbose
 
 ROLLUP_BIN        = rollup
-ROLLUP_OPTS       = --config rollup.config.js
+ROLLUP_OPTS       = --config rollup.config.mjs
 
 TRANSCRYPT_BIN    = transcrypt
 TRANSCRYPT_OPTS   = --build --map --nomin --sform --esv 6

README.md

@@ -60,7 +60,7 @@ Both can be stored locally to use OOMAnalyser without an Internet connection.
 ```
 # . env/bin/activate
 # transcrypt --build --map --nomin -e 6 OOMAnalyser.py
-# rollup --config rollup.config.js
+# rollup --config rollup.config.mjs
 
 or